A band-gap reference is a circuit that creates a voltage reference that is constant across process variation, supply voltage, and temperature. These circuits are used to generate output voltages of regulators, reference voltages for input/output circuits, precise biasing signals, and any other application requiring a constant voltage reference. Some band-gap references generate a current reference as well to be used in biasing circuits to provide desired operating points.
Referring to FIG. 2, a conventional band-gap reference circuit is shown providing a Vbg output reference signal. Band-gap reference circuits typically comprise current sources or comparators that force the output of a band-gap reference circuit higher than some low convergence point. The band-gap reference circuit has multiple convergence points, meaning that the band-gap reference circuit might have multiple output voltages that it can become trapped at. For example, a typical band-gap voltage reference produces a 1.2V output. Other circuits are then designed to use this 1.2V output. However, during power up of a bang-gap circuit, the output might also have a lower convergence point such as a 0.4V output voltage and become trapped at this value. This may be due, for example, to certain offset conditions in the feedback loop of the error amplifier of the band-gap reference circuit.
A startup circuit is used to guarantee the band-gap reference circuit does not get trapped at a lower convergence point, and the start up circuit pulls the band-gap reference circuit output up to the correct level (e.g., 1.2V). Two examples of conventional band-gap startup circuits are shown in FIGS. 1A-B and provide a startup current for use in a band-gap reference circuit.
Both startup circuits in FIGS. 1A-B operate under the principle that the startup current ‘Istartup’ is connected to some node in the band-gap reference circuit having multiple convergence points. If the node in the band-gap reference circuit is low (which will be the case during start-up), current is sourced into the node until it reaches a desired level and turns off the sourcing device (i.e., NMOS transistors Q4, Q6 in FIGS. 1A-B).
As recognized by the present inventor, disadvantages of conventional band-gap startup circuits include that their output current (used to startup the band-gap reference circuit) can vary with process, voltage and temperature (PVT). As a result of these variations, conventional band-gap startup circuits do not always guarantee startup for a band-gap reference circuit. For example, the current ‘Istartup’ in FIGS. 1A-B can shut-off before the band-gap reference circuit to which it is connected reaches its desired operating point. Furthermore, as recognized by the present inventor, if the startup current does not turn off after the ban-gap circuit is at its desired operating point, the extra current from the startup circuit can cause the band-gap reference circuit to provide an incorrect output Vbg.
As recognized by the present inventor, what is needed is a circuit and method for startup of a band-gap reference circuit that provides startup signals substantially independent of PVT variations or fluctuations.
It is against this background that various embodiments of the present invention were developed.